We know that asteroids can do a lot of damage. One barreled into the Yucatan Peninsula 66 million years ago and caused the demise of 70 percent of all life on Earth, including the dinosaurs. Within moments the six-mile-wide chunk of rock, known as the Chicxulub impactor, caused tsunamis so large that the Great Plains were overwhelmed with water.
Giant shock waves reverberated across the planet, clouding the sky with volcanic eruptions and shaking the ground with endless earthquakes. What’s worse, no one saw this mega-disaster coming, which leaves us to wonder: could it happen again? What’s out there, and do we know if it’s headed our way?
What’s the Difference Between a Meteor and an Asteroid?
We hear these terms interchangeably all the time, but to truly understand what’s coming at us, you have to know the difference. According to Davide Farnocchia, a navigation engineer at NASA’s Jet Propulsion Laboratory in Pasadena, California, the difference between a meteor and an asteroid is all about location. “An asteroid is in outer space, and it becomes a meteor when it goes through the Earth’s atmosphere,” he says.
Both asteroids and meteors are made up of silicates, often containing more iron and nickel than most rocks found on the surface of the Earth. A meteoroid results when a chunk of rock breaks off from a larger asteroid (usually a result of impact) and burns up on its way into the Earth’s atmosphere. As it burns up, it lights up in the sky, and that’s why it’s referred to as a shooting star.
Once a meteor is in the Earth’s atmosphere, it’s here, and there’s not much we can do about it, says Carson Fuls, a senior operations engineer at the Catalina Sky Survey at the University of Arizona. “The goal is to find asteroids many years before they have any probability of impacting Earth,” says Fuls.
The Largest Asteroids
One of the largest asteroids in the asteroid belt is 4 Vesta. While it’s enormous at 525 km across, there’s no danger that it will impact Earth. Fun fact: it’s one of the few large enough to have a crust, mantle and a core.
Fuls says that we know where 98 percent of the largest asteroids floating in space are located. The rocks, which number around 800, are at least a kilometer or more in size. “These are the ones that are big enough to devastate the entire biosphere,” he says.
We can’t know for sure that we’ve seen every enormous asteroid in space because there could always be a huge one in the wrong place at the wrong time, such that our telescopes continue to miss it. But this is very unlikely.
Most Interesting Asteroids
(Credit:NASA/Goddard/University of Arizona)
This image shows four views of asteroid Bennu along with a corresponding global mosaic. The images were taken on Dec. 2, 2018, by the OSIRIS-REx spacecraft’s PolyCam camera.
One of the most famous asteroids to make the rounds in space in recent years is Asteroid 99942, known as Apophis. It is around 340 meters across and would cause widespread regional damage — is set to make a very close approach to Earth in 2029.
It will be close enough to be seen with the naked eye, “like a point of light crossing the sky.” Fuls says that when it was discovered in 2004, there was initially some concern that it could hit Earth. “Since that time, we’ve re-observed it many, many times, and we’ve confirmed that there’s a zero percent chance that it will impact Earth,” he says.
101955 Bennu is another interesting asteroid, though it’s much smaller than Apophis. Every six years it passes close to Earth, and as a result, scientists have been very interested in studying it.
On October 20, 2020, NASA’s OSIRIS-REx spacecraft landed on the asteroid’s surface and took a sample of the rock, which was found to be more carbon-rich than other asteroids, says Farnocchia. And according to NASA, “its materials are so old, Bennu may contain organic molecules similar to those that could have been involved with the start of life on Earth.”
Dangerous Types of Asteroids
According to experts, it’s not the big ones that we should be worried about. It’s the smaller types of asteroids that would be large enough to cause regional devastation. These are around 140 meters or more, and they’re much harder to spot in space because they’re more numerous. Fuls says that we know the whereabouts of about 40 percent of these smaller-sized objects. According to NASA, the current known asteroid count is 1,284,478.
For scientists like Farnocchia and Fuls, spotting and tracking asteroids is a labor of love. Fuls looks for ones that are near-Earth full-time, a job which he says brings him sheer joy. Six nights every lunar month, Fuls spends all night behind a telescope. He feels lucky that he’s one of the experts tasked with discovering asteroids that no one has ever seen before. These giant rocks, says Fuls, “they literally keep me up all night.”